In a long term evolution advanced (LTE-A, fourth generation mobile communication) frequency division duplex (FDD) type, a fixed relay node (RN), which shares the same frequency on an Uu interface representing an interface between the RN and a user equipment (UE) and an Un interface representing an interface between a base station (eNB) and the RN and is located in a region in which incoming and outgoing signals are not isolated, is called as a Type1 RN. The Type1 RN uses a demodulation reference signal (DMRS) for demodulation upon a relay physical downlink control channel (R-PDCCH). The Type1 RN considers resource wastage and robustness, which are occurred for the R-PDCCH to perform an effective resource mapping of the R-PDCCH, and performs interleaving on a basis of a physical resource block (PRB) for the R-PDCCH, which is different from a release 8 (REL-8) PDCCH.
FIG. 1 is a schematic diagram showing backhaul signaling in the Type1 RN. As shown in FIG. 1, in the case of the Type1 RN, backhaul data and control data are exchanged between the RN and a donor eNB through time division multiplexing (TDM) based half duplexing to avoid a self interference (SI). The donor eNB represents a base station including a relay node.
A data transmission from the eNB to the RN is performed by using a specific subframe. The subframe is transmitted from the RN to the UEs through a system information block (SIB) as a multimedia broadcasting single frequency network (MBSFN) subframe. Furthermore, a cell specific reference signal (CRS), a physical hybrid-ARQ indicator channel (PHICH) and a physical control format indicator channel (PCFICH) for measuring channel quality information (CQI) of the UEs existing within the coverage of the RN are transmitted on a control part 10 of the MBSFN subframe and the PDCCH should be transmitted except a downlink (DL) grant. Furthermore, there is no data transmission on a data part 20 of the MBSFN sub frame for the UEs existing within the coverage of the RN and only data received from the eNB are transmitted on the data part 20 of the MBSFN subframe.
A subframe number for the Un interface, a frequency location in the subframe for the R-PDCCH and a start symbol location of the R-PDSCH are transmitted by using radio resource control (RRC) signaling that is set once and then intermittently reset by the donor eNB for each RN. The DL grant for the RN is always located at a first slot of the subframe and an uplink (UL) grant may be located at a second slot of the same PRB including the DL grant, if the UL grant exists.
At release 10 (REL-10), the DMRS is a reference signal separable for each of the UEs, unlike the CRS, which has been adopted for demodulation of the PDSCH in a transmission of a higher order multiple input and multiple output (MIMO) (8*8). The DMRS signals of the first and second slots are code division multiplexed (CDM). Accordingly, the DMRS signals should be located in the same PRB, in which the PDSCH is located, for the demodulation of the PDSCH. When this is applied to the R-PDCCH, if the UL grant for the RN exists in the PRB, in which the DL grant for the RN is located, then the UL grant must be always located in the second slot of the PRB. The UL grant can't be transmitted through multiple PRBs for acquisition of frequency diversity, unlike the CRS transmitting a common signal value through all frequency bands for demodulating the PDCCH of a Macro UE. Eventually, the R-PDCCH should be mapped on a basis of the PRB identical to the DMRS based PDSCH. That is to say, if the DMRS based R-PDCCH was transmitted to all frequency bands through a resource element group (REG) based interleaving, then it may not be possible to demodulate the R-PDCCH based on the DMRS by the RN. Accordingly, the R-PDCCH for a specific RN should be located in the PRB for the R-PDCCH previously transmitted for the specific RN, when the R-PDCCH based on the DMRS is applied.
At REL-8, a size of a control channel element (CCE) for the PDCCH is defined as 9 REGs. Signals of the REL-8 are transmitted by applying an aggregation level of 1, 2, 4 or 8 according to a link quality of the UE, and the signals of the REL-8 are received by using a blind detection method at the UE. That is to say, the signals are interleaved on a basis of the REG size at all frequency bands for transmission such as 9 REGs at the aggregation level of 1, 18 REGs at the aggregation level of 2, 36 REGs at the aggregation level of 4 and 72 REGs at the aggregation level of 8. Although the signals exceed the REG size acceptable in one PRB, all system bandwidths are interleaved at an interleaving size, so that there is no problem to acquire the frequency diversity.
This issue has been discussed many times, however, it has been decided to define the 9 REGs as the 1CCE size in the DMRS or CRS based R-PDCCH. Prior to this decision, there was an proposal suggesting the use of a 16-quadrature amplitude modulation (QAM) in the case of the RN instead of quadrature phase shift keying (QPSK), which is used for the PDCCH of a frequently moving UEs at the REL-8, under the condition that sufficient backhaul radio quality is provided by fixedly positioning the Type1 RN in the LTE-A. However, when the 16 QAM is used at downlink control information (DCI) formats, which may be expected to be used frequently for the R-PDCCH, a redundancy and a padding overhead may be considerably increased against real information. Furthermore, because all the Type1 RNs on a cell don't have best backhaul quality of service (QoS), a decoding success rate of the R-PDCCH for a specific RN declines noticeably when using the 16 QAM, which may be negatively affected to a capacity of whole network. If a carrier aggregation is applied to the RN, then a carrier indication field (CIF) of three bits may be added to almost all the DCI formats and it may not be easy to predict what kinds of information elements are added to the DCI format except for the CIF. FIG. 2 is a schematic diagram showing a DMRS based R-PDCCH for the Type1 RN. At the 3GPP, even if there is waste of a resource due to some redundancies and padding overhead in applying the DMRS based R-PDCCH, such as the REG regions indicated by lattices in FIG. 2, a discussion about a change of the REG size itself within a category minimizing a change of a REL-8 specification has been stopped. Considering the above, a symbol number of PDCCH for the UL grant is increased to seven symbols such that the symbol number for the DL grant becomes asymmetric with the symbol number for the UL grant. The information element, which is added to the DCI format for the Macro UE of the REL-10 considering the 3GPP specification and a forward compatibility, is nothing but size increasing of the DCI format for the CIF and the UL single user MIMO (SU-MIMO). The purpose of the addition of the information element is minimum impact of the specification and simplification of the specification for adoption of a fast RN to the REL-10.
As shown in FIG. 2, because the 9 REGs are not included in a single PRB, a repetition and a padding are additionally performed at the last region of the R-PDCCH through 2 PRBs, when there is no change of the CCE size of the DL grant defined at the 3GPP. Because the Type1 RN is fixed, the repetition and the padding are treated as waste of the resources, when the radio quality of the UE of the Type1 RN is better than that of a normal UE in some degree. Because the repetition and the padding are performed in the case of the UL grant as same as the DL grant, the repetition and the padding are causing waste of the resources.
A donor eNB, which has whole frequency bandwidths less than 10 PRBs, can't have the REL-10 RN. The DCI format size, which will be increased by adoption of the UL SU-MIMO, may not over seventy bits defined at the REL-8 for the DL SU-MIMO based on 100 PRB. As shown in FIG. 4, the DCI of seventy-three bits, which is maximum size at the REL-8/9, will be mapped to 37 resource elements (REs), i.e. 10 REG at the REL-10 through the QPSK. As shown in FIG. 2, the PRB has shortage of 2 REG in the DMRS based R-PDCCH, especially a case of the DL grant. So there is a problem that another PRB is needed for accommodation of the 2 REGs.